Diadenosine polyphosphate-activated inward and outward currents in follicular oocytes of Xenopus laevis.

Ionic currents evoked by alpha, omega-adenine dinucleotides (ApXA; X = 2-6) in follicular oocytes of Xenopus laevis were studied under voltage-clamp conditions. Dinucleotides evoked inward and outward currents in Xenopus oocytes by activating native P1 and P2 purinoceptors known to be present on the follicle cell monolayer enveloping oocytes. Inward currents were mediated by a suramin-sensitive P2 purinoceptor which showed an agonist potency order (at 10 microM): Ap4A > ATP > Ap3A > > Ap5A, while Ap2A and Ap6A were inactive. Outward currents were mediated by a novel theophylline-sensitive P1 purinoceptor which showed an agonist potency order (at 10 microM): Ap2A > ATP > > Ap4A = Ap5A = Ap6A > Ap3A. Chromatographic analysis confirmed ectonucleotidase activity at the follicle cell layer of oocytes but at a very low rate of dinucleotide cleavage, indicating that currents evoked by dinucleotides resulted from a direct activation of oocyte P1 and P2 purinoceptors and not through their breakdown to ATP, ADP and AMP. There was no evidence for specific receptors (i.e., P4 purinoceptors) for diadenosine polyphosphates in Xenopus oocytes.